1. Field of the Invention
The invention relates to an engine characteristic estimation method for estimating a steady state value of an engine characteristic that changes in accordance with an engine control parameter, and a computer readable medium that stores a computer executable code which executes an engine characteristic estimation method for estimating a steady state value of an engine characteristic that changes in accordance with an engine control parameter.
2. Description of the Related Art
In a commonly-used engine, it is preferable to control engine control parameters such as ignition timing and an air-fuel ratio (a fuel injection amount) in each engine operating state, which is defined by an engine speed and an engine load, to achieve optimum engine characteristics such as engine output power, exhaust emission, exhaust gas temperature, and catalytic device temperature. In order to execute this control, target values of the engine control parameters in each engine operating state need to be set in advance.
To set the target values, a conformance test is executed in the following manner using, for example, a prototype engine. In each engine operating state, a specific engine control parameter is changed with the other engine control parameters fixed. Then, a change in a specific engine characteristic is monitored, whereby a specific engine control parameter value at which the specific engine characteristic is at the optimum value is determined.
In this conformance test, the engine characteristic, which is subject to monitoring, does not change to the steady state value promptly in response to a change in the specific engine control parameter. If each value of the specific engine control parameter remains unchanged until the specific engine characteristic, which is subject to monitoring, reaches the steady state value corresponding to the value of the specific engine control parameter, a considerably long test time is required until the specific engine characteristic, which is subject to monitoring, reaches the steady state value. If this process is performed for all of the engine operating states, a huge amount of time is required to complete the conformance test.
In order to reduce the conformance test time in each engine operating state, the following method is suggested in, for example, Japanese Patent Application Publication No. 2002-206456 (JP-A-2002-206456). According to this method, a specific engine control parameter is varied within an estimated range, and several steady state values of a specific engine characteristic are measured for the corresponding several values of the specific engine control parameter. Then, a model equation is set based on these several steady state values of the specific engine control parameter, and the steady state values of the specific engine characteristic are estimated to correspond to other values of the specific engine control parameter.
However, the values of the specific engine characteristic corresponding to the other values of the specific engine control parameter are not taken into account in the estimation of the steady state values of the specific engine characteristic, which correspond to the other values of the specific engine control parameter. Therefore, the reliability of the estimated steady state values is not very high.
For example, a steady state value of the specific engine characteristic may be estimated in the following method. In a specific engine operating state, a change in a specific engine characteristic, which is caused when the specific engine control parameter is changed, is modeled based on a recurrence equation model (for example, an ARX model) in which a post-change specific engine control parameter is used. Then, several values of the specific engine characteristic are measured after the specific engine control parameter is changed. In this way, the steady state value of the specific engine characteristic is estimated, which corresponds to the post-change specific engine control parameter. According to this estimation method, it is no longer necessary to maintain the post-change specific engine control parameter until the specific engine characteristic becomes the steady state value. Therefore, the conformance test time is reduced. In addition, the reliability of the estimated steady state value of the specific engine characteristic is high, because the value of the specific engine characteristic, which is obtained when the specific engine control parameter is changed, is taken into account in the estimation.
In the specific engine operating state, if the specific engine control parameter is changed to a first value from when the specific engine characteristic is at a steady state value and a first steady state value of the specific engine characteristic is estimated as described above, which corresponds to the first value of the specific engine control parameter, the reliability of the estimated first steady state value is high.
However, if the value of the specific engine control parameter is changed from a first value to a second value, and then from a second value to a third value, it is not possible to accurately estimate the steady state value of the specific engine characteristic (which corresponds to the second value of the specific engine control parameter) during the period in which the value of the specific engine control parameter is changed from the second value to the third value, even if the value of the specific engine control parameter during this period is directly substituted into the above-described ARX model. This is because the value of the specific engine characteristic during this period is influenced by the engine operation that is performed when the specific engine control parameter is at the first value.